Method for bit error rate measurements in a cell-based telecommunication system

ABSTRACT

A method for bit error rate measurements in a cell-based telecommunication system is presented, comprising the following steps of generating a first bit pattern at a first location of said telecommunication system, transmitting said first bit pattern from said first location to a second location of said telecommunication system, thereby obtaining a transmitted bit pattern, generating a second bit pattern at said second location, said second bit pattern being identical with the first bit pattern and comparing said transmitted bit pattern with said second bit pattern. The first bit pattern is transmitted in at least one unused cell (U, I), said unused cell (U, I) being generated during normal operation of said telecommunication system in the case of a lack of used cells to be transmitted.

BACKGROUND OF THE INVENTION

The present invention relates to a method for bit error ratemeasurements in a cell-based telecommunication system.

In cell-based telecommunication systems, the information is transferredby means of cells of fixed or variable byte-length. These cellstypically have an overhead section wherein control, management androuting information is embedded and a payload section, carrying userinformation.

Cell-based telecommunication systems comprise for example ATM(Asynchronous Transfer Mode) communication networks. ATM cells forinstance have a fixed length of 53 bytes, 8 bytes of which constitutethe overhead section or the so called ATM cell header, reserved foroverhead information.

The whole transmitted cell-stream in a cell-based network is generallycomposed of two different types of cells: on the one hand used cellscarrying in their payload section the proper user information and on theother hand unused cells with no user information in their payloadsection. The unused cells are generated and inserted amid the used cellsto be transmitted in order to provide a continuous cell-stream betweentransmitting means and receiving means.

Bit errors which occur during transmission of these cells, have anegative impact onto the quality of the connection. A bit error whichoccurs in the payload section of the cell implies a falsification of thetransmitted information; wherein a bit error which occurs in theoverhead section of the cell potentially implies a falsification of thedestination address, and thus a loss of the cell.

Therefore, in cell-based telecommunication systems, the bit error rates(ratio of bits transferred erroneously due to noise or impairments ofthe physical transmission medium in proportion to the totality oftransferred bits) are traditionally measured during initialisation ofthe system or during a test phase. Hereby a predetermined signal is sentfrom a transmitter to a receiver. The arriving signal is then analysedat the receiver's side by comparing the transmitted signal with agenerated reference signal.

In an ADSL (Asymmetric Digital Subscriber Line) system for instance, thesignal to noise ratio (SNR) is measured at the different carrierfrequencies during initialisation of the system. This signal to noiseratio can be seen as a measure for the expected bit error rate. Theresults of these measurements afterwards are used to determine the bitallocations, i.e. the way wherein data bits are distributed over thedifferent carriers that constitute a DMT (Discrete Multi Tone) symbol,to cope with the maximum allowable bit error rate of 10⁻⁷ prescribed bythe ADSL standard. The SNR measurements and the use thereof in the bitallocation process are described in the ANSI (American NationalStandards Institute) ADSL Standard T1E1.4, in paragraph 12 entitled‘Initialization’ on pages 87-111 and in paragraph 6.5 entitled ‘ToneOrdering’ on Pages 36-37.

Traditional bit error rate measurement methods have the disadvantage,that normal operation of the system has to be interrupted and the systemhas to be brought in a test phase. During this test phase, notransmission of proper user information is possible.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for bit errorrate measurements in a cell-based telecommunication system during normaloperation of the system.

According to the invention, this problem is solved by a method for biterror rate measurements in a cell-based telecommunication system inaccordance to claim 1.

The method for bit error rate measurements in a cell-basedtelecommunication system, comprises the steps of generating a first bitpattern at a first location of the telecommunication system,transmitting the first bit pattern from the first location to a secondlocation of the telecommunication system, thereby obtaining atransmitted bit pattern, generating a second bit pattern at the secondlocation, the second bit pattern being identical with the first bitpattern and comparing the transmitted bit pattern with the second bitpattern. According to the invention the first bit pattern is transmittedin at least one unused cell, the unused cell being generated duringnormal operation of the telecommunication system in the case of lack ofused cells, carrying user information, to be transmitted. The number ofdiscrepancies counted during the comparison serves as a measure for thebit error rate.

It has to be noted, that in this context, the expression “unused cell”means cells, which do not carry user data in their payload section. Inother words “unused cell” can either designate an idle cell or anunassigned ATM cell.

One essential advantage of the invention, compared with conventionalmethods is that the bit error rate measurements are performed duringnormal operation of the telecommunication system. Accordingly thetelecommunication system has not to be interrupted and brought in a testphase in which no transmission of proper user information is possible.

In a cell-based telecommunication system, e.g. an ATM-network, whereunused cells, carrying no user information, are generally inserted amidthe used cells in order to provide a continuous cell-stream betweentransmitting means and receiving means, bit error rate measurementsaccording to the present method advantageously need no additionalbandwidth by using those unused cells.

It has to be noted, that the generated bit patterns are independent fromany transmitted user information and that the bit pattern generated atthe first location and the reference bit pattern generated at the secondlocation are identical. This means, that the number of detecteddiscrepancies between the transmitted bit pattern and the second bitpattern really represents a measure for the bit error rate at the momentof transmission, because the bit errors only can occur in the test bitpattern itself. This is not the case in systems, where controlinformation, calculated on the basis of user data to be transmitted, istransmitted together with said user information and the transmittedcontrol information is compared to control information calculated on thebasis of the transmitted user data. In fact, in these systems, biterrors can occur as well in the transmitted user data as in thetransmitted control information, thus leading to a possiblefalsification of the measurement results.

In a preferred embodiment, the steps for bit error rate measurementsaccording to the present method are repeated at each generation of anunused cell. The accuracy of the achieved bit error rates is increasingwith the number of repetitions, i.e. with the number of comparisonsbetween transmitted bit patterns and generated bit patterns at thereceiver.

The comparison between the transmitted bit pattern and the second bitpattern is preferably bit synchronised, which enables a fast evaluationof bit error rates during measurements.

The generation of the second bit pattern may be synchronised with thearrival of the transmitted bit pattern at a comparator means forcomparing said transmitted bit pattern with said second bit pattern.

The first bit pattern is advantageously a predetermined bit sequence oran algorithm-based counter bit-sequence. Both types of bit patterns maybe easily reproduced at the receiving means.

The present invention also relates to a device for bit error ratemeasurement in a cell-based telecommunication system. A device accordingto the invention comprises a transmitter, a receiver, a first bitpattern generator for generating a first bit pattern to be transmittedfrom the transmitter to the receiver in order to obtain a transmittedbit pattern, a second bit pattern generator for generating a second bitpattern at the receiving means, the second bit pattern being identicalto the first bit pattern, a comparator for comparing the transmitted bitpattern with the second bit pattern and means for inserting the firstbit pattern in at least one unused cell, said unused cell beinggenerated during normal operation of said telecommunication system inthe case of lack of used cells to be transmitted.

In a preferred embodiment, the device for bit error rate measurementscomprises a synchroniser for activating the second bit pattern generatorat each arrival of an unused cell at said comparator.

In the case of a bi-directional communication system, each subscriberstation is simultaneously sending and receiving cells, thus providedwith transceivers, each transceiver comprising transmitting means andreceiving means.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more apparent from the followingdescription of a not limiting embodiment with reference to the attacheddrawings, wherein

FIG. 1 shows a schematic view of a cell-transfer between a firsttransceiver and a second transceiver in a cell-based telecommunicationnetwork,

FIG. 2 shows a schematic view of a transmitter of a device for bit errormeasurements;

FIG. 3 shows a schematic view of a receiver of a device for bit errormeasurements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic communication link between a first transceiverTRX1 and a second transceiver TRX2 in a cell-based network, e.g. anATM-Network.

both transceivers TRX1 and TRX2 are interconnected via a physicaltransmission medium, for example an optical fibre cable, or via awireless transmission path represented here by link L.

The information transferred between the first transceiver TRX1 and thesecond transceiver TRX2 is packed in cells of fixed or variable length.These cells typically have an overhead section wherein control,management and routing information is embedded, and a payload section,carrying user information.

Cell-based telecommunication systems comprise for example ATM(Asynchronous Transfer Mode) communication networks. ATM cells forinstance have a fixed length of 53 bytes, 8 bytes of which constitutethe overhead section or the so called ATM cell header, reserved foroverhead information.

The cells are transmitted via link L either from the first transceiverTRX1 to the second transceiver TRX2. In the case of a bi-directionalcommunication system, each transceiver TRX1 and TRX2 is simultaneouslysending and receiving cells. Accordingly, each of the transceivers TRX1and TRX2 comprises a transmitter for sending and a receiver forreceiving cells.

The whole transmitted cell-stream in a cell-based network is generallycomposed of two different types of cells: on the one hand used cellscarrying in their payload section the proper user information and on theother hand unused cells with no user data in their payload section. Theunused cell can be either an idle cell I or an unassigned cell U.

FIG. 2 shows a schematic view of a transmitter TX, component oftransceiver TRX1 and transceiver TRX2 in an ATM-network. The transmitterTX comprises an ATM-transmitter circuitry ATM-TC for converting anarriving bit stream of user information at the input IN into usedATM-cells, an unused cell generator UCG for generating unused cells inthe case of a lack of used ATM-cells to be transmitted and a multiplexerMUX for collecting the used ATM-cells and the unused cells to a commonand continuous cell-stream at the output O. The multiplexer MUX maycomprise a waiting queue (not shown) for storing cells, for example aFIFO-queue (First In First Out-queue).

A first bit pattern generator PG1 is present at the transmitter TX forgenerating a first reproducible bit pattern and for inserting saidreproducible bit pattern into unused cells generated in said unused cellgenerator UCG. It has to be noted that the first bit pattern generatorPG1 is advantageously integrated into the unused cell generator UCG.

FIG. 3 shows a schematic view of a receiver RX, component of transceiverTRX1 and transceiver TRX2 in an ATM-network. The receiver RX comprises ademultiplexer DMUX for splitting up the arriving cell-stream at theinput IN′ in used ATM-cells and unused cells. An ATM-receiving circuitryATM-RC is connected to the demultiplexer DMUX for delineating the usedcells in the received bit stream and for decoding the proper userinformation at the output O1. The receiver further comprises asynchroniser SYN for activating a second bit pattern generator PG2 witheach arrival of an unused cell I, U, whereby the second bit patterngenerator PG2 is generating a copy of the reproducible bit pattern, anda comparator CMP for comparing the transmitted bit pattern with the bitpattern generated the second bit pattern generator PG2 at the outputO2′.

During the transmission of user information in an ATM-network from thefirst transceiver TRX1 to the second transceiver TRX2, the userinformation to be transmitted, which is present in a bit stream form atthe input IN, is converted by the ATM-transmitter circuitry ATM-TC intoATM-cells. Thereby the user bits are embedded in the payload section ofthe ATM-cells, forming the so called used cells. The overhead section orheader of each used ATM-cell carries a destination address—in this casethe address of the transceiver TRX2—and routing information.

On transmit, the used cells are mapped by the multiplexer MUX into aTime Division Multiplexing (TDM) frame format. Generally ATM-networksare used for the simultaneous transmission of multiple cell streamsbelonging to several services, for example video, moved pictures, sound,and data, whereby the multiple cell streams are multiplexed together inthe multiplexer MUX. In the example represented in the figures however,the transmitter TX only transmits one single cell stream (only oneATM-transmission circuitry ATM-TC).

During the multiplexing process, if an ATM-slot is not immediatelyavailable, the arriving used ATM-cell at the input of the multiplexerMUX is stored in the waiting queue (not shown) of the multiplexer MUX.Otherwise, if there is no used ATM-cell to transmit and if the queue ofthe multiplexer MUX is empty when the time arrives to fill the nextsynchronous cell time slot, then an unused cell U, I, generated by theunused cell generator UCG, is inserted instead of a used ATM-cell. Inother words, those unused cells U, I are generally inserted amid theused ATM-cells so as to provide a continuous cell-stream between a firstand a second transceiver, TRX1 and TRX2. By sending unused cells U, I,an ATM-network performs cell rate decoupling when it is not providedwith used cells. This function allows an ATM-network to operate with awide range of different speed physical interfaces, i.e. to operate witha wide range of different services.

According to the present invention, these unused cells I, U are used forbit rate measurements during normal operation of the system, i.e. duringtransmission of used ATM-cells for instance from a transmitter TX of thefirst transceiver TRX1 to a receiver RX of the second transceiver TRX2.

A first bit pattern generator PG1 generates a first reproducible bitpattern and inserts said first bit pattern into the payload section ofan unused cell U, I generated by the unused cell generator UCG of thetransmitter TX. In the case of lack of used ATM-cells at the multiplexerMUX, the unused cell U, I carrying the bit pattern is inserted by themultiplexer MUX amid used ATM-cells in the cell-stream.

Both types of cells, the used ATM-cells (carrying user information)generated by the ATM-transmission circuitry and the unused ATM-cells U,I (carrying reproducible bit patterns) generated by the unused cellgenerator UCG, are collected to a common and continuous cell-stream atthe output O of the multiplexer MUX and sent via link L (FIG. 1) to thereceiver RX (FIG. 3) of the second transceiver TRX2 (FIG. 1).

On the reception at the input IN′ of the demultiplexer DMUX, thearriving cell-stream is split up the demultiplexer DMUX on the one handinto the used ATM-cells and on the other hand into the unused ATM-cellsI, U (FIG. 3). Both types of cells are distinguished by means of theiroverhead section, carrying a different identification pattern.

The used ATM-cells are relayed to the ATM-receiving circuitry ATM-RC,where the individual cells are delineated in the received bit stream anddecoded in the proper user information.

On arrival of a cell, identified as an unused cell U, I, the second bitpattern generator PG2 of the receiver RX is activated. To make thesystem work properly, the operation of the second bit pattern generatorPG2 and the comparator CMP in the receiver RX of the second transceiverTRX2 needs to be synchronised. This is the task of the synchroniser SYN,which activates the second pattern generator PG2 with each new arrivalof an unused cell U, I, i.e. with the arrival of the transmittedpredetermined bit pattern at the comparator CMP.

The second bit pattern generator PG2 generates bit-by-bit a bit pattern,identical to the reproducible bit pattern inserted in the payloadsection of that cell U, I by the unused cell generator UCG of thetransceiver TX. Simultaneously the comparator CMP compares bit-by-bitthe transmitted bit pattern carried in the payload section of thearrived cell U, I with the reproduced bit pattern generated by thesecond bit pattern generator PG2.

The number of discrepancies counted during comparison between thetransmitted bit patterns and the copied bit patterns serve as a measurefor the bit error rate.

In this way, statistical bit error rate measurements are performedduring normal operation of the telecommunication system, which has notto be interrupted and brought in a test phase.

The bit error rate measurements according to the present inventionoccupy no additional bandwidth on the link L between the transceiversTRX1, TRX2 and the required additional equipment in the transceiversTRX1, TRX2 is simple. The unused cell generator UCG of the transmitterTX just needs to be able to embed in a payload section of an unused cellU, I a reproducible pattern, that will be transmitted and then used forstatistical bit error rate measurements.

1. A method for bit error rate measurements in a cell-basedtelecommunication system, comprising: a) generating a first bit patternat a first location of said telecommunication system, b) transmittingthe first bit pattern from said first location to a second location ofsaid telecommunication system, thereby obtaining a transmitted bitpattern, c) generating a second bit pattern at said second location,said second bit pattern being identical with the first bit pattern, andd) comparing said transmitted bit pattern with said second bit pattern,wherein said first bit pattern is transmitted in at least one unusedcell, said unused cell being generated, without the need to enter adedicated test mode, during normal operation of said telecommunicationsystem in the case of lack of used cells to be transmitted.
 2. Themethod according to claim 1, wherein the steps of a)-d) are repeated ateach generation of an unused cell.
 3. The method for bit error ratemeasurements according to claim 1, wherein the comparison between saidtransmitted bit pattern and said second bit pattern is bit synchronized.4. The method for bit error rate measurements according to claim 1,wherein the generation of the second bit pattern is synchronized withthe arrival of the transmitted bit pattern at a comparator means forcomparing said transmitted bit pattern with said second bit pattern. 5.The method for bit error rate measurements according to claim 1, whereinsaid first bit pattern is a predetermined bit pattern or analgorithm-based counter bit-sequence.
 6. A device for bit error ratemeasurements in a cell-based telecommunication system, comprising:transmitting means, receiving means, a first bit pattern generator forgenerating a first bit pattern to be transmitted from said transmittingmeans to said receiving means, thereby obtaining a transmitted bitpattern, a second bit pattern generator for generating a second bitpattern at said receiving means, said second bit pattern being identicalto said first bit pattern, a comparator means for comparing saidtransmitted bit pattern with said second bit pattern, and means forinserting said first bit pattern into at least one unused cellgenerated, without the need to enter a dedicated test mode, duringnormal operation of said telecommunication system in the case of lack ofused cells to be transmitted.
 7. The device for bit error ratemeasurements according to claim 6, further comprising a synchronizer foractivating said second bit pattern generator at an arrival of an unusedcell at said comparator means.
 8. A device for bit error ratemeasurements in a cell-based telecommunication system, comprising:transmitting means; receiving means; a first bit pattern generator forgenerating a first bit pattern to be transmitted from said transmittingmeans to said receiving means, thereby obtaining a transmitted bitpattern; a second bit pattern generator for generating a second bitpattern at said receiving means, said second bit pattern being identicalto said first bit pattern; an unused cell generator for generatingunused cells; a comparator means for comparing said transmitted bitpattern with said second bit pattern; and means for inserting said firstbit pattern into at least one said unused cell, which is beinggenerated, without the need to enter a dedicated test mode, duringnormal operation of said telecommunication system in the case of lack ofused cells to be transmitted, wherein said first bit pattern isintegrated into said unused cell generator.
 9. The device for bit errorrate measurements in a cell-based telecommunication system as set forthin claim 8, further comprising a synchronizer for activating said secondbit pattern generator.
 10. The device for bit error rate measurements ina cell-based telecommunication system as set forth in claim 8, whereinsaid transmitted bit pattern is synchronized with said second bitpattern at said comparator means.
 11. The device for bit error ratemeasurements in a cell-based telecommunication system as set forth inclaim 8, wherein cells generated by said unused cell generator arecombined with used cells at an output of a multiplexer.
 12. A method forbit error rate measurements in a telecommunication system, comprising:a) transmitting a sequence of bits having a transmitted bit pattern,from a first location to a second location of said telecommunicationsystem, b) receiving said sequence of bits at said second location as areceived bit sequence having a received bit pattern, and c) at saidsecond location, comparing said received bit pattern with a referencebit pattern identical to said transmitted bit pattern, wherein saidtransmitted sequence of bits is inserted into at least one unused cell,said unused cell being generated during normal operation of saidtelecommunication system in the case of lack of used cells to betransmitted, without a need to enter a dedicated test mode.
 13. Themethod according to claim 12, wherein the steps of a)-c) are repeated ateach generation of an unused cell.
 14. The method for bit error ratemeasurements according to claim 12, wherein the comparison between saidreceived bit pattern and said reference bit pattern is bit synchronized.15. The method for bit error rate measurements according to claim 12,wherein said reference bit pattern is generated at said second locationin synchronism with the arrival of the received bit pattern at acomparator means for comparing said received bit pattern with saidreference bit pattern.
 16. The method for bit error rate measurementsaccording to claim 12, wherein said transmitted bit pattern is apredetermined bit pattern or an algorithm-based counter bit-sequence.17. A transmitting unit for use in a telecommunication system,comprising: a transmitter for providing a first bit pattern and fortransmitting said first bit pattern to a receiving unit, to be comparedfor bit error rate measurements with a second bit pattern provided atsaid receiving unit and identical to said first bit pattern, whereinsaid transmitter further includes an insertion unit for inserting saidfirst bit pattern into at least one unused cell generated during normaloperation of said telecommunication system in the case of lack of usedcells to be transmitted, without the need to enter a dedicated testmode.
 18. The transmitting unit according to claim 17, wherein saidfirst bit pattern is a predetermined bit pattern or an algorithm-basedcounter bit-sequence.
 19. The transmitting unit according to claim 17,wherein unused cells generated by said transmitter are combined withused cells at an output of a multiplexer.
 20. A receiving unit for usein a telecommunication system, comprising: a receiver for providing afirst bit pattern and for receiving a transmitted bit pattern from atransmitting unit, and including a comparator for comparing said firstbit pattern with said transmitted bit pattern for bit error ratemeasurements, wherein said receiver further includes an extraction unitfor extracting said transmitted bit pattern from at least one unusedcell generated during normal operation of said telecommunication systemin the case of lack of used cells to be transmitted, without a need toenter a dedicated test mode.
 21. The receiving unit according to claim20, wherein the comparison between said first bit pattern and saidtransmitted bit pattern is bit synchronized.
 22. The receiving unitaccording to claim 20, wherein the receiver further comprises asynchronizer for activating the providing of said first bit pattern atthe arrival of said transmitted bit pattern at said comparator.
 23. Thereceiving unit according to claim 20, wherein said first bit pattern isa predetermined bit pattern or an algorithm-based counter bit-sequence.